The present invention is directed to an external optical resonator for a semiconductor laser which is composed of an optical directional coupler comprising two optical waveguides wherein the optical power beamed out of the semiconductor laser through an end face of one of the two waveguides can be coupled into the directional coupler and can be taken from the other end face of this one waveguide and whereby the optical feedback means is arranged at every end face of the other optical waveguide of the two waveguides, said optical feedback means at least partially coupling optical power supplied from the other waveguide back to the other waveguide.
Single-mode, narrow-band laser transmitters are required for the realization of coherently optical transmission systems. Fabry-Perot semiconductor lasers have a multi-mode structure. In combination with external resonators, narrow-band, single-mode laser transmitters can be realized from such a semiconductor laser and are suitable for optical communication systems having heterodyne or homodyne reception (see the article from Brit. Telecom. Techn. Journal, Vol. 3, No. 4, 1985).
European Application A2-0 216 212 discloses an external optical resonator that is composed of an optical directional coupler comprising two optical waveguides wherein the light powered beam-out by the semiconductor laser can be coupled into the directional coupler through an end face of one of the two waveguides. A feedback means is provided at the other end face of this one waveguide, and this feedback means feeds a slight part of the optical power supplied in the one waveguide back into the one waveguide but couples the majority part of this optical power out of this one waveguide.
The out-coupled optical power supplied through a fiber to an end face of another waveguide of the directional coupler and is coupled into this other waveguide. An optical power coupled out from the other end face of the other waveguide is supplied either to a radiation absorber or is coupled into a system fiber as an output power of the semiconductor laser. In this last embodiment, the system fiber and the resonator are coupled to the same end face of the laser. In comparison to the embodiment wherein the resonator and the system fiber are coupled to different end faces of the laser, this has the advantage that the mechanical outlay is reduced.
The optical directional couplers can be composed of an optical fiber directional coupler or of an integrated directional coupler. A second feedback means that feeds a slight part of the optical power supplied in the fiber back into the fiber can be arranged at the other port of the other waveguide of the directional coupler.
An external optical resonator in the form of a fiber-optical directional coupler comprising two optical waveguides wherein it is possible to couple the resonator and the system fiber to the same end face of the semiconductor laser is proposed in U.S. Pat. No. 4,799,234, which claims priority from German Application 36 33 077 and whose disclosure is herein by reference thereto. This frequency-selective fiber directional coupler is composed of a dispersive double-core fiber, wherein the two cores form the two optical waveguides of the directional coupler. One end face of one core is coupled to the semiconductor laser and the end face of the other core is arranged at the same side as this one end face is free of any connection. A feedback means is arranged at the other end faces of both cores and this feedback means feeds a slight part of the optical power supplied in the one core back into this one core and couples the other part out of the core. The feedback means has no further function with respect to the other core. The optical power coupled out from the one core is coupled into a system fiber.
U.S. Pat. No. 4,831,631, whose disclosure is incorporated herein by referenc thereto and which claims priority from German Application 36 33 076, proposes external resonators for semiconductor lasers that are constructed with integrated-optical directional couplers and wherein the system fiber and the resonator are coupled to the same end face of the semiconductor laser. The two optical waveguides of the directional coupler are strip waveguides integrated on a substrate. The optical power emitted by the semiconductor laser is coupled into one strip waveguide and a feedback means is provided at the other end face of this waveguide. This feedback means couples the optical power supplied from this one waveguide into this waveguide. A feedback means is provided at the end face of the other strip waveguide that is arranged at the same side as the other end face of the one waveguide, and this feedback means feeds the optical power supplied in the other waveguide back into this other waveguide. The system fiber is coupled to the other end face of the other waveguide. In order to achieve a single-mode operation, it is important that the two feedback means are arranged at different optical spacings from a coupling path of the directional coupler. The coupling path of the directional coupler is defined by the distance in which its two waveguides are conducted at such a slight distance from one another, side-by-side, that a coupling of power between the two waveguides will occur.